JP6152782B2 - Hot rolled steel sheet - Google Patents

Description

  The present invention relates to a steel plate, and more particularly to a hot-rolled steel plate.

  Steel sheets used in general automobiles typified by passenger cars and large industrial transport vehicles typified by trucks are required to be lighter and stronger.

  It is difficult to press-mold a high-strength steel plate having a tensile strength of 590 MPa or more. Then, when manufacturing a flame | frame and a gate using such a high intensity | strength hot-rolled steel plate, the multistage forming (roll forming) using a roll is implemented. In roll forming, steel materials (frame, gate, etc.) are formed by performing multi-stage bending. Therefore, bending workability is required for a high-strength hot-rolled steel sheet.

  Also, when shearing high-strength hot-rolled steel sheets, the clearance is widened in order to reduce the shear load and reduce the consumption of the mold (punch and die). For example, the clearance is 5 to 30% of the plate thickness. By the way, in a hot-rolled steel plate, a thick plate having a thickness of 6 mm or more may be used. If the thick plate as described above is sheared with a wide clearance, defects are likely to occur on the end face. In particular, in a high-strength thick plate, a crack along the plate surface may occur on the end surface after shearing. Such cracks reduce the fatigue durability of the steel sheet. Therefore, the hot-rolled steel sheet may be required to have characteristics (hereinafter referred to as shear workability) in which the above-described cracks hardly occur even when sheared.

  Japanese Unexamined Patent Publication No. 2009-144225 (Patent Document 1) and Japanese Unexamined Patent Application Publication No. 2009-179852 (Patent Document 2) propose steel sheets with improved bendability.

  Patent Document 1 describes the following matters. The microstructure of the steel sheet is a structure in which ferrite and martensite are uniformly and finely dispersed. Specifically, the microstructure includes 50% or more of ferrite and 10% or more of martensite.

  Patent Document 2 describes the following matters. The microstructure of the steel sheet contains 30% or more of ferrite and 30 to 70% martensite phase by area ratio. The area ratio of tempered martensite to all martensites is 20% or more, and the area ratio of martensite having a particle diameter of 1 μm or less to all martensites is 10% or less.

  However, the microstructures of the steel sheets of Patent Documents 1 and 2 contain 10% or more of martensite, which is a hard phase. Therefore, the bendability may be low.

JP 2009-144225 A JP 2009-179852 A

  An object of the present invention is to provide a hot-rolled steel sheet having high strength and excellent bendability.

The hot-rolled steel sheet according to the present embodiment has a chemical composition and a microstructure. Chemical composition is mass%, C: 0.01-0.2%, Si: 0.01-2.5%, Mn: 0.5-3.0%, P: 0.02% or less, S : 0.005% or less, Sol. Al: 0.02-0.5%, Ti: 0.02-0.25%, N: 0.010% or less, Nb: 0-0.1%, V: 0-0.4%, Mo: 0 to 0.3%, W: 0 to 0.3%, Cr: 0 to 0.3%, total content of Ca, Mg and rare earth elements (REM): 0 to 0.01%, The balance consists of Fe and impurities. The microstructure consists of ferrite and bainite with a total area ratio of 89% or more, pearlite with an area ratio of 0 to 5%, martensite with an area ratio of 0 to 3%, and retained austenite with an area ratio of 0 to 3%. It consists of. The hot-rolled steel sheet has a tensile strength of 590 MPa or more. The Vickers hardness HvC at the center of the thickness of the hot-rolled steel sheet and the Vickers hardness HvS at a position 100 μm deep from the plate surface satisfy the formula (1).
HvS / HvC ≦ 0.80 (1)

  The hot-rolled steel sheet according to this embodiment has high strength and excellent bendability.

The hot-rolled steel sheet according to the present embodiment has a chemical composition and a microstructure. Chemical composition is mass%, C: 0.01-0.2%, Si: 0.01-2.5%, Mn: 0.5-3.0%, P: 0.02% or less, S : 0.005% or less, Sol. Al: 0.02-0.5%, Ti: 0.02-0.25%, N: 0.010% or less, Nb: 0-0.1%, V: 0-0.4%, Mo: 0 to 0.4%, W: 0 to 0.4%, Cr: 0 to 0.4%, total content of Ca, Mg and rare earth elements (REM): 0 to 0.01%, The balance consists of Fe and impurities. The microstructure consists of ferrite and bainite with a total area ratio of 89% or more, pearlite with an area ratio of 0 to 5%, martensite with an area ratio of 0 to 3%, and retained austenite with an area ratio of 0 to 3%. It consists of. The hot-rolled steel sheet has a tensile strength of 590 MPa or more. The Vickers hardness HvC at the center of the thickness of the hot-rolled steel sheet and the Vickers hardness HvS at a position 100 μm deep from the plate surface satisfy the formula (1).
HvS / HvC ≦ 0.80 (1)

  In the hot-rolled steel sheet according to the present embodiment, the Vickers hardness ratio (= HvS / HvC) becomes 0.80 or less by containing Ti and performing the manufacturing method described later. That is, the hardness of the surface layer of the hot-rolled steel sheet is lower than that at the center of the sheet thickness. Therefore, even if it is a high-strength hot-rolled steel sheet having a tensile strength of 590 MPa, excellent bendability can be obtained.

  Preferably, when the thickness of the hot-rolled steel sheet is tmm, the {112} <110> orientation and the crystal orientation of 10 ° or less are provided at the center of the thickness within the range of ± 0.12 t from the thickness center. The ratio of the total area of crystal colonies having a {112} <110> orientation and a crystal orientation of 10 ° or less and an aspect ratio of 0.35 or less to the total area of the crystal colonies is 40% or less.

  In the center of the plate thickness, a crystal colony having a {112} <110> orientation and a crystal orientation of 10 ° or less and an aspect ratio of 0.35 or less is defined as a “band colony”. If the ratio of the total area of the band-like colonies to the total area of the crystal colonies at the center of the plate thickness (hereinafter referred to as the band-like colony area ratio) is high, the shear processability is lowered. In the hot-rolled steel sheet of the present embodiment, the total area ratio of band-like colonies is 40% or less. Therefore, even if it is a high-strength hot-rolled steel sheet having a tensile strength of 590 MPa, excellent shear workability can be obtained.

  The chemical composition of the above hot-rolled steel sheet is Nb: 0.002-0.1%, V: 0.01-0.4%, Mo: 0.01-0.4%, W: 0.01-0. .4% and Cr: one or more selected from the group consisting of 0.01 to 0.4% may be contained.

The chemical composition of the above hot-rolled steel sheet may contain 0.0002 to 0.01% in total of one or more selected from the group consisting of Ca, Mg, and REM.
Hereinafter, the hot-rolled steel sheet of this embodiment will be described in detail. “%” Of the content of each element means “mass%”.

[Chemical composition]
The chemical composition of the hot-rolled steel sheet according to the present embodiment contains the following elements.

C: 0.01 to 0.2%
Carbon (C) increases the strength of the steel sheet. If the C content is too low, it is difficult to obtain a tensile strength of 590 MPa or more. On the other hand, if the C content is too high, coarse carbides are formed at the grain boundaries, and the workability of the steel sheet decreases. Therefore, the C content is 0.01 to 0.2%. The minimum with preferable C content is higher than 0.01%, More preferably, it is 0.02%, More preferably, it is 0.03%. The upper limit with preferable C content is less than 0.2%, More preferably, it is 0.18%, More preferably, it is 0.15%.

Si: 0.01 to 2.5%
Silicon (Si) suppresses the formation of coarse iron carbide and improves bendability. If the Si content is too low, this effect cannot be obtained. On the other hand, if the Si content is too high, the above effect is saturated. If the Si content is too high, the weldability of the steel sheet further decreases. Therefore, the Si content is 0.01 to 2.5%. The minimum with preferable Si content is higher than 0.01%, More preferably, it is 0.2%, More preferably, it is 0.5%. The upper limit with preferable Si content is less than 2.5%, More preferably, it is 2.0%, More preferably, it is 1.5%.

Mn: 0.5 to 3.0%
Manganese (Mn) increases the strength of the steel sheet. If the Mn content is too low, it is difficult to obtain a tensile strength of 590 MPa or more. On the other hand, if the Mn content is too high, ferrite transformation after finish rolling is delayed, and it becomes difficult to produce ferrite having high formability. Therefore, the Mn content is 0.5 to 3.0%. The minimum with preferable Mn content is higher than 0.5%, More preferably, it is 0.8%, More preferably, it is 1.0%. The upper limit with preferable Mn content is less than 3.0%, More preferably, it is 2.5%, More preferably, it is 2.2%.

P: 0.02% or less Phosphorus (P) is an impurity. P segregates at the grain boundaries and embrittles the steel sheet. Therefore, P decreases bendability and shear workability. Therefore, the P content is preferably as low as possible. The P content is 0.02% or less. The P content is preferably less than 0.02%, more preferably 0.01% or less, and still more preferably 0.007% or less.

S: 0.005% or less Sulfur (S) is an impurity. S combines with Mn, Ti and the like to form coarse sulfide inclusions. Sulfide inclusions promote cracks in the cut section. Sulfide inclusions further reduce the workability of the steel sheet. Accordingly, the S content is preferably as low as possible. S content is 0.005% or less. A preferable S content is less than 0.005%, more preferably 0.002% or less, and still more preferably 0.001% or less.

Sol. Al: 0.02 to 0.5%
Aluminum (Al) promotes ferrite transformation and enhances the formability of the steel sheet. Further, Al suppresses the formation of coarse cementite and suppresses the formation of an end face crack starting point. If the Al content is too low, the above effect cannot be obtained. On the other hand, if the Al content is too high, the austenite-ferrite transformation temperature becomes too high, and the productivity is lowered. Therefore, Sol. The Al content is 0.02 to 0.5%. Sol. The minimum with preferable Al content is higher than 0.02%, More preferably, it is 0.08%, More preferably, it is 0.11%. Sol. The upper limit with preferable Al content is less than 0.5%, More preferably, it is 0.3%, More preferably, it is 0.25%. The Al content referred to in this specification is Sol. It means the content of Al (acid-soluble Al).

Ti: 0.02-0.25%
Titanium (Ti) combines with C to form carbonitride, and increases the strength of the steel sheet by fine precipitation hardening. Furthermore, Ti carries out the manufacturing method mentioned later, and makes the hardness of the plate | board thickness center part of a hot-rolled steel plate higher than a surface layer. If the Ti content is too low, the above effect cannot be obtained. On the other hand, if the Ti content is too high, coarse carbonitrides are produced and formability of the steel sheet is lowered. If the Ti content is too high, the above effect is further saturated and the raw material cost is increased. Therefore, the Ti content is 0.02 to 0.25%. The minimum with preferable Ti content is higher than 0.02%, More preferably, it is 0.05%, More preferably, it is 0.08%. The upper limit with preferable Ti content is less than 0.25%, More preferably, it is 0.2%, More preferably, it is 0.17%.

N: 0.010% or less Nitrogen (N) is an impurity. N combines with Ti, Nb, V, etc. to form coarse nitrides. Therefore, the cut end face crack is promoted, and the shear workability is lowered. Accordingly, the N content is preferably as low as possible. N content is 0.010% or less. The preferable N content is less than 0.010%, more preferably 0.005% or less, and further preferably 0.003% or less.

  The balance of the chemical composition of the hot rolled steel sheet according to the present embodiment is composed of Fe and impurities. Here, the impurities are those that are mixed from ore as a raw material, scrap, or production environment when industrially producing a steel material, and do not adversely affect the hot-rolled steel sheet of the present embodiment. Means what is allowed.

  The chemical composition of the hot-rolled steel sheet according to the present embodiment may further contain Nb instead of a part of Fe.

Nb: 0 to 0.1%
Niobium (Nb) is an optional element and may not be contained. When contained, Nb forms carbonitride and refines austenite grains. Thereby, the nucleation site of a ferrite increases and the coarsening of a steel structure is suppressed. Nb is further contained together with V or Ti to form fine precipitates. Fine precipitates increase the strength of the steel sheet. However, if the Nb content is too high, the formation of a band structure is promoted and cracks at the cut end face tend to occur. Therefore, the Nb content is 0 to 0.1%. The minimum with preferable Nb content is 0.002%, More preferably, it is 0.006%, More preferably, it is 0.008%. The upper limit with preferable Nb content is less than 1.0%, More preferably, it is 0.05%, More preferably, it is 0.003%.

  The chemical composition of the hot-rolled steel sheet according to the present embodiment further contains one or more selected from the group consisting of V, Mo, W, and Cr instead of part of Fe.

V: 0 to 0.4%,
Mo: 0 to 0.4%,
W: 0 to 0.4%
Cr: 0 to 0.4%
Vanadium (V), molybdenum (Mo), tungsten (W), and chromium (Cr) are all optional elements and may not be contained. When contained, all of these elements combine with C to form fine carbides. These fine carbides increase the strength of the steel sheet by fine precipitation hardening. However, if the content of these elements is too high, the effect is saturated and the raw material costs are increased. Therefore, the content of each element is 0 to 0.4%. The lower limit of the content of each element is preferably 0.01%, more preferably 0.05%, and further preferably 0.08%. The upper limit of each element is preferably less than 0.4%, more preferably 0.35%, and still more preferably 0.30%.

  The chemical composition of the hot-rolled steel sheet according to the present embodiment further contains one or more selected from the group consisting of Ca, Mg, and REM instead of a part of Fe.

Total content of Ca, Mg and REM: 0 to 0.01%
Calcium (Ca), magnesium (Mg) and rare earth element (REM) are all optional elements and may not be contained. When contained, these elements form oxides in the molten steel. Thereby, steel is deoxidized and the cleanliness of steel increases. Furthermore, since oxides form carbonitride nuclei, the posture of coarse carbonitrides is suppressed and the occurrence of cut end face cracks is suppressed. However, if the content of these elements is too high, coarse oxides are formed, and the cleanliness and formability of the steel are reduced. Therefore, when 1 type or 2 types or more are contained from the group which consists of Ca, Mg, and REM, the total content of these elements is 0 to 0.01%. The minimum with preferable total content of these elements is 0.0002%, More preferably, it is 0.0005%, More preferably, it is 0.0010%. The upper limit with preferable total content of these elements is less than 0.01%, More preferably, it is 0.0050%, More preferably, it is 0.0030%.

  REM in the present specification contains at least one of Sc, Y, and lanthanoid (La at atomic number 57 to Lu at 71), and the REM content means the total content of these elements. To do.

[Microstructure]
In the microstructure of the hot-rolled steel sheet according to the present embodiment, the total area ratio occupied by ferrite and bainite is 89% or more. Further, pearlite having an area ratio of 5% or less, martensite having an area ratio of 10% or less, and retained austenite having an area ratio of 3% or less may be contained.

Ferrite, or ferrite and bainite: 89% or more in total The total area ratio of ferrite and bainite in the microstructure of the hot-rolled steel sheet is 89% or more. In this case, the bendability and shear formability of the hot-rolled steel sheet are enhanced. A preferable lower limit of the total area ratio of ferrite and bainite is 90%, and more preferably 95%. The total area ratio of ferrite and bainite may be 100%, or the ferrite area ratio may be 100% (that is, a ferrite single phase). That is, the area ratio of bainite may be 0%.

Perlite: 0-5%
Pearlite becomes a starting point of cracking at the time of cutting and causes cracking of the cut surface. Therefore, the area ratio of pearlite in the microstructure is preferably low. The area ratio of pearlite is 0 to 5%. A preferable upper limit of the area ratio of pearlite is 3%. The most preferable area ratio of pearlite is 0%.

Martensite: 0-3%
Martensite decreases bendability. Further, martensite becomes a starting point of cracking at the time of cutting and causes a crack at the cut end face. Therefore, the area ratio of martensite in the microstructure is preferably low. The area ratio of martensite is 0 to 3%. A preferable upper limit of the area ratio of martensite is 1%. The most preferable martensite area ratio is 0%.

Residual austenite: 3% or less Residual austenite becomes a starting point of cracking at the time of cutting and causes cracking at the cut end face. Furthermore, the retained austenite is transformed into martensite at the time of punching, and the hole expandability is lowered. Accordingly, the area ratio of retained austenite in the microstructure is preferably low. The area ratio of retained austenite is 0 to 3%. A preferable upper limit of the area ratio of retained austenite is 1%. The most preferable area ratio of retained austenite is 0%.

[Measurement method of area ratio of each phase in microstructure]
The area ratio of each phase (ferrite, bainite, pearlite, martensite, retained austenite) is obtained by the following method.

  A sample including a position (observation position) at a depth of t / 2 (t is a plate thickness) is taken from the surface of the hot-rolled steel sheet. The surface including the observation position (observation surface) is etched by night. Among the observed observation surfaces, the tissue observation is performed on any 10 visual fields (each visual field area is 200 μm × 200 μm). A 500 × optical microscope is used for tissue observation, and a 1000-2000 × scanning electron microscope (SEM) is used for phase identification as necessary.

  By observing the structure, the area ratio (%) of ferrite and bainite, the area ratio (%) of pearlite, the area ratio (%) of martensite, and the area ratio (%) of retained austenite in each visual field are obtained. And the average of the area ratio of each phase obtained in each field of view, the area ratio (%) of ferrite and bainite in the microstructure of the hot-rolled steel sheet in this embodiment, the area ratio (%) of pearlite, the martensite The area ratio (%) and the area ratio (%) of retained austenite are defined.

[Vickers hardness ratio]
The hot-rolled steel sheet of this embodiment further satisfies the following formula (1).
HvS / HvC ≦ 0.80 (1)
Here, HvC is the Vickers hardness at the thickness center position (t / 2 position) of the hot-rolled steel sheet. HvS is the Vickers hardness at a position (hereinafter referred to as a surface layer position) at a depth of 100 μm from the surface of the hot-rolled steel sheet.

  That is, in the hot-rolled steel sheet of the present embodiment, the Vickers hardness at the surface layer position is lower than the Vickers hardness at the sheet thickness center position. In this case, cracks are unlikely to occur on the steel sheet surface due to bending. Therefore, the hot-rolled steel sheet has excellent bendability.

[Measurement method of Vickers hardness ratio]
The Vickers hardness at the plate thickness center position and the surface layer position is measured by the following method. A Vickers hardness test in accordance with JIS Z2244 (2009) is performed at an arbitrary 10 positions in the center of the plate thickness and at an arbitrary 10 positions in the surface layer position. The test force is 1.0 kgf = 9.8 N.

  An average of 10 Vickers hardnesses at the plate thickness center position is defined as Vickers hardness HvC at the plate thickness center position. The average of 10 Vickers hardnesses at the surface layer position is defined as the Vickers hardness HvS at the surface layer position. Preferably, HvS / HvC is less than 0.80.

  If the manufacturing method mentioned later is implemented, the Vickers hardness at the surface layer position can be made smaller than the Vickers hardness at the plate thickness center position.

[Band colony area ratio BCR]
Crystal colonies having a crystal orientation of 10 ° or less with respect to the {112} <110> orientation within a range of ± 0.12 t from the plate thickness center of the hot-rolled steel plate to the plate thickness direction (hereinafter referred to as plate thickness center portion). Is defined as AC0. Further, within the above range, among the crystal colonies having a crystal orientation of 10 ° or less with respect to {112} <110>, the total area of crystal colonies having an aspect ratio of 0.35 or less (hereinafter referred to as band-like colonies) Is defined as AC1. At this time, the band-like colony area ratio BCR (%) defined by the formula (2) is preferably 40% or less.
BCR = AC1 / AC0 × 100 (2)

  Crystal colonies are also called orientation colonies. In this specification, a crystal colony means a group of a plurality of crystal grains in which adjacent crystal grains have a crystal orientation of 10 ° or less with respect to the {112} <110> orientation.

The aspect ratio is defined by the following formula (3).
Aspect ratio = Crystal colony minor axis / Crystal colony major axis (3)
Here, the short axis and long axis of the crystal colony are defined as follows. The boundary of each crystal colony specified by EBSD described later is approximated (fitted) to an ellipse by the least square method. The obtained elliptical short and long axes are defined as the short and long axes of the crystal colony.

  In the present embodiment, as described above, the band-like colony area ratio BCR defined by the formula (2) is preferably 40% or less. In this case, the shear processability is improved, and cracks at the cut end face are suppressed.

  The {112} <110> crystal orientation is the ferrite transformation orientation from the unrecrystallized austenite. Therefore, a crystal colony having a crystal orientation of 10 ° or less with respect to the {112} <110> orientation tends to form a crystal colony (band-like colony) extending in a band shape along the rolling direction. In other words, it is easy to form an elongated band-like colony having an aspect ratio defined by the above formula (3) of 0.35 or less.

  A band-like colony is likely to cause a cut end face when it is cut parallel to the rolling direction. Therefore, the area ratio BCR (%) of the band-like colony is preferably low.

  As described above, when the band-like colony area ratio BCR defined by the formula (2) is 40% or less, the occurrence of cracks at the cut end face is suppressed, and the shear workability is improved. A more preferable band-like colony area ratio BCR is 30% or less.

[Measurement method of area ratio BCR of band-like colony]
The area ratio BCR of the band-like colony is measured by the following method. The hot-rolled steel sheet is cut in the sheet thickness direction along the rolling direction. The cut surface extends along the rolling direction. The cut surface is mechanically polished to a mirror surface. Thereafter, the mirror surface is subjected to mechanical chemical polishing using colloidal silica to remove the polishing scratches on the mirror surface to obtain an observation surface. Among the observation surfaces, a sample including an observation surface portion within a range of ± 1/2 t (plate thickness center portion) in the plate thickness direction from the plate thickness center of the hot-rolled steel plate is collected. Crystal grain orientation analysis is performed on the observation surface of the sample by EBSD analysis.

  The observation magnification is 200 times. The observation area is 700 μm × 700 μm, and analysis is performed at 1 μm intervals. Through analysis, the crystal orientations of a plurality of crystal grains in the observation region are specified.

Based on the analysis result, a crystal colony having a crystal orientation of 10 ° or less with respect to the {112} <110> orientation is specified in the observation region. The total area AC0 (μm ² ) of the plurality of identified crystal colonies is obtained.

Further, the major axis and minor axis of the plurality of identified crystal colonies are measured to determine the aspect ratio. A crystal colony having an aspect ratio of 0.35 or less is identified as a band-like colony. The total area AC1 (μm ² ) of the identified plurality of band-like colonies is obtained. Then, using the total areas AC0 and AC1, the band-like colony area ratio BCR (%) is obtained based on the equation (2).

[Coarse colony area ratio CCR]
When the area ratio BCR of the band-shaped colony is 40% or less, more preferably, within the thickness center portion of the hot-rolled steel sheet (in the range of ± 1/2 t from the thickness center of the hot-rolled steel sheet to the thickness direction). , {112} Of the crystal colonies having a crystal orientation of 10 ° or less with respect to the <110> orientation, the area ratio of crystal colonies (hereinafter referred to as coarse colonies) having an equivalent circle diameter of 10 μm or more (hereinafter referred to as coarse colony area ratio) CCR) is 30% or less.

Specifically, the total area of crystal colonies having a crystal orientation of 10 ° or less with respect to the {112} <110> orientation is AC0 (μm ² ), and the total area of coarse crystal colonies is AC2 (μm ² ). In some cases, the coarse colony area ratio CCR (%) is defined by equation (4).
Coarse colony area ratio CCR = AC2 / AC0 × 100 (4)
The equivalent circle diameter means the diameter (μm) of a circle when the area of the specified crystal colony is converted into a circle having the same area.

  If the coarse colony area ratio CCR is high, cracks generated from coarse inclusions, crystallized substances, and the interface between the precipitate and the parent phase are likely to propagate. This is because the grain boundary in the colony is considered to be a small-angle grain boundary (boundary where the crystal orientation angle with the adjacent crystal grain is less than 15 °), and the grain boundary in the colony does not work sufficiently as a barrier for crack propagation. it is conceivable that.

  When the band-like colony area ratio BCR is 40% or less and the coarse colony area ratio CCR is 30% or less, the shear workability of the hot-rolled steel sheet is further enhanced. Furthermore, the area ratio CCR of a preferable coarse colony is 25% or less.

[Measurement Method of Coarse Colony Area Ratio CCR]
The area ratio of coarse colonies is measured by the following method. A plurality of crystal colonies are identified in the observation region under the same measurement conditions and measurement method as the method for measuring the band-like colony area ratio BCR. The total area AC0 (μm ² ) of the plurality of identified crystal colonies is obtained.

Further, the equivalent circle diameter of each identified crystal colony is obtained to identify a coarse colony. The total area AC2 (μm ² ) of the identified coarse colony is determined. And the area ratio (%) of a coarse colony is calculated | required based on Formula (4) using total area AC0 and AC2.

[Characteristics of hot-rolled steel sheet]
The tensile strength of the hot-rolled steel sheet having the above chemical composition and microstructure is 590 MPa or more. Preferably, the yield ratio is 0.75 or more. The hot-rolled steel sheet of this embodiment has excellent bendability despite having high strength.

[Production method]
An example of the manufacturing method of the above-mentioned hot-rolled steel sheet will be described. The manufacturing method of a hot-rolled steel sheet includes a step of preparing a slab (slab preparation step), a step of heating the slab (heating step), and a step of rolling the heated slab into a hot-rolled steel plate ( A rolling process), a process of cooling the hot-rolled steel sheet (cooling process), and a process of winding the hot-rolled steel sheet after cooling (winding process). Hereinafter, the manufacturing conditions of each process are explained in full detail.

[Slab preparation process]
A molten steel having the above chemical composition is produced. Slabs are manufactured using molten steel. The slab may be manufactured by a continuous casting method. The ingot may be manufactured using molten steel, and the ingot may be subjected to ingot rolling to manufacture a slab.

[Heating process]
Heat the slab. For example, the slab is charged in a heating furnace or a soaking furnace and heated. The preferred heating temperature of the slab is 1100-1300 ° C. If the heating temperature is too low, the Ti carbonitride in the slab remains without being dissolved. In this case, the strength of the steel sheet is lowered. On the other hand, if the heating temperature is too high, the yield decreases. Therefore, the heating temperature is preferably 1100 to 1300 ° C. A preferable heating time (soaking time) at the heating temperature is 1 hour or more.

[Rolling process]
The heated slab is hot-rolled using a rough rolling mill and a finish rolling mill to produce a hot-rolled steel sheet. The rough rolling mill includes a plurality of roll pairs arranged in a row. The finish rolling mill also includes a plurality of roll pairs arranged in a row.

[Total rolling reduction MFR]
Preferably, in the rolling step, the total rolling reduction MFR is 35% or more when the surface temperature of the steel plate (slab) during rolling is between 1100 ° C and 1000 ° C. The total rolling reduction MFR (%) is defined as shown in Equation (5).
MFR = (plate thickness of steel plate at 1100 ° C. (mm) −plate thickness of steel plate at 1000 ° C. (mm)) / plate thickness of steel plate at 1100 ° C. (mm) × 100 (5)

  If the total rolling reduction MFR is too low, coarse colonies and band-like colonies are likely to be generated. If a large number of coarse colonies and band-like colonies are generated, the cut end face cracks are likely to occur. Therefore, the total rolling reduction MFR is 35% or more. A preferable total rolling reduction MFR is 40% or more.

  If the total rolling reduction MFR exceeds 30%, the coarse colony area ratio CCR exceeds 30%, but the band-like colony area ratio BCR becomes 40% or less. Therefore, good shear workability can be obtained. However, if the total rolling reduction MFR is 35% or more, excellent shear workability can be obtained.

[Total rolling reduction TFR]
Furthermore, the total rolling reduction TFR from the surface temperature of the steel plate during rolling to 1000 ° C. until the rolling at the stand immediately before the final stand is 10 to 80%.

  If the total rolling reduction TFR is too high, a large number of band-like colonies are formed, and the band-like colony area ratio BCR exceeds 40%. On the other hand, if the total rolling reduction TFR is too low, shape defects may occur in the hot-rolled steel sheet. Therefore, the total rolling reduction TFR is 10 to 80%. A preferable upper limit of the total rolling reduction TFR is 75%, and more preferably 70%.

[Final rolling reduction FR, friction coefficient μ in final rolling]
Furthermore, the rolling reduction (final rolling reduction) FR at the final stand of the finish rolling mill is 20 to 50%. Furthermore, the friction coefficient μ between the rolling roll in the final rolling (that is, the roll of the final stand) and the steel plate being rolled is 0.20 or more.

  When the final rolling reduction FR is 20% or more and the friction coefficient μ is 0.20 or more, a large strain is applied to the surface layer of the hot-rolled steel sheet at the time of final rolling. In this case, the ferrite nose shifts to the short time side (high temperature side) in the CCT curve. Therefore, at the surface layer position of the hot-rolled steel sheet, TiC precipitates and grows at an earlier stage than the plate thickness center position. Therefore, when fine TiC is deposited at the center of the plate thickness, the TiC at the surface layer has already grown and is larger than the TiC at the center of the plate thickness.

  Since TiC at the surface layer position is larger than TiC at the sheet thickness center position, the hardness of the surface layer of the hot-rolled steel sheet is lower than the sheet thickness center part. Specifically, the Vickers hardness HvS at the surface layer position satisfies the formula (1) with respect to the Vickers hardness HvC at the plate thickness center position. Therefore, the bendability of the hot rolled steel sheet is increased.

  When the final rolling reduction FR is less than 20%, or when the friction coefficient μ is less than 0.20, the above effect cannot be obtained. On the other hand, if the final rolling reduction FR is too high, a large number of band-like colonies are generated, and the band-like colony area ratio exceeds 40%. For this reason, the shear processability is lowered.

  A preferable lower limit of the final rolling reduction FR is higher than 20%, and more preferably 25%. A preferable upper limit of the final rolling reduction is less than 50%, and more preferably 40%. A preferable friction coefficient μ is 0.25 or more.

  The coefficient of friction μ can be adjusted, for example, by controlling the amount of lubricant applied to the surface of the roll pair of the final stand per unit time. Further, the smaller the roll diameter, the higher the friction coefficient. Therefore, the friction coefficient may be adjusted according to the roll diameter of the roll pair used for the final stand. Further, the friction coefficient may be adjusted depending on the material of the roll.

[Finishing rolling temperature FT]
In hot rolling, the surface temperature of the steel sheet on the exit side of the final stand of the finish rolling mill is defined as the finish rolling temperature (° C.). In the present embodiment, the finish rolling temperature FT is 930 to 1100 ° C. If finish rolling temperature FT is too low, many band-shaped colonies will produce | generate and a band-shaped colony area ratio will exceed 40%. For this reason, the shear processability is lowered. On the other hand, if finish rolling temperature FT is too high, productivity will fall. Therefore, finish rolling temperature FT is 930-1100 degreeC. A preferable lower limit of the finish rolling temperature is 970 ° C.

[Cooling process]
The hot-rolled steel sheet after hot rolling is air-cooled for 3 to 10 seconds. After air cooling, the hot-rolled steel sheet is cooled (forced cooling) at a cooling rate CR of 15 to 200 ° C./second. Preferably, the cooling stop temperature ST during forced cooling is set to 400 to 750 ° C. A residence time from when forced cooling is stopped to when winding is performed may be provided for 2 to 15 seconds. There may be no residence time. Preferably, the winding temperature during winding is 400 to 600 ° C. Hereinafter, each condition will be described in detail.

[Air cooling time after hot rolling]
The air cooling time after hot rolling is 3 to 10 seconds. By setting the air cooling time to 3 seconds or more, static recrystallization occurs and the generation of band-like colonies is suppressed. On the other hand, if the air cooling time is too long, coarse Ti carbonitride is formed, and the strength of the steel sheet is lowered. Therefore, the air cooling time after hot rolling is set to 3 to 10 seconds.

[Forced cooling]
After the air cooling time has elapsed, the hot-rolled steel sheet is cooled at a cooling rate CR of 15 to 200 ° C./second. If the cooling rate CR is too slow, ferrite precipitates coarsely at high temperatures. Furthermore, Ti carbonitride becomes coarse and the strength of the hot-rolled steel sheet decreases. On the other hand, if the cooling rate is excessively high, excessive cooling equipment is required, resulting in an increase in manufacturing cost. Further, productivity may be hindered by excessive cooling equipment. Therefore, the cooling rate is 15 to 200 ° C./second. A preferable lower limit of the cooling rate is 20 ° C./second. A preferable upper limit of the cooling rate is 100 ° C./second. The forced cooling at the cooling rate is, for example, water cooling.

[Cooling stop temperature ST]
The cooling stop temperature ST for forced cooling (the surface temperature of the hot-rolled steel sheet when forced cooling is stopped) is preferably 400 to 750 ° C. If the cooling stop temperature is too low, a hard phase (such as martensite) is likely to be formed. On the other hand, if the cooling stop temperature is too high, the Ti carbonitride becomes coarse and the strength of the hot-rolled steel sheet tends to decrease. Therefore, a preferable cooling stop temperature is 400 to 750 ° C.

[Residence time]
The residence time from when forced cooling is stopped to when winding is started is preferably 2 to 15 seconds. If the residence time is 2 seconds or more, the ferrite is stably precipitated. Therefore, the balance of strength, elongation and hole expansibility of the hot-rolled steel sheet is improved. On the other hand, if the residence time is too long, the Ti carbonitride becomes coarse. For this reason, the strength of the hot-rolled steel sheet is reduced. Accordingly, the preferred residence time is 2 to 15 seconds. A more preferred lower limit of the residence time is 5 seconds, and a more preferred upper limit is 10 seconds. The residence time may not be provided.

[Winding process]
After forced cooling, the hot rolled steel sheet is wound (coil winding). The surface temperature (winding temperature CT) of the hot-rolled steel sheet at the start of coil winding is preferably 400 ° C to 600 ° C. When the coiling temperature CT is less than 400 ° C., the strength of the steel sheet is obtained by generating a low temperature transformation phase represented by martensite. However, when the strength increases due to the generation of the low temperature transformation phase, the difference between the strength of the surface layer of the steel sheet and the strength of the central portion is small. When the coiling temperature CT is less than 400 ° C., a retained austenite phase is easily obtained. Since the retained austenite phase is a metastable phase, it tends to change into a martensite phase by processing. From the above, when the winding temperature CT is less than 400 ° C., the bendability is lowered.

  When coiling temperature CT is 400-600 degreeC, the intensity | strength of a steel plate is obtained by precipitation strengthening (precipitation of TiC). In this case, since the ferrite is generated at a high temperature by shearing in the surface layer portion, TiC becomes coarse. On the other hand, in the center portion, ferrite is generated at a lower temperature than the surface layer portion, so that TiC does not coarsen and precipitates finely. Therefore, the strength of the surface layer portion can be made lower than that of the central portion, and the bendability of the steel sheet is increased.

  On the other hand, when the coiling temperature CT exceeds 600 ° C., the TiC at the center is coarsened. Therefore, the difference between the strength of the surface layer and the strength of the central portion is reduced, and the bendability of the steel sheet is reduced.

  A more preferable upper limit of the coiling temperature CT is 500 ° C. In this case, the toughness of the steel sheet is increased, and the occurrence of cut end face cracks is suppressed. That is, the cutability of the steel sheet is increased.

  Molten steel having the chemical composition shown in Table 1 was produced.

  With reference to Table 1, the chemical composition of steel DH, M, and N was in the range of the chemical composition of the hot-rolled steel sheet of this embodiment. On the other hand, the chemical compositions of Steels A to C and I to L were outside the range of the chemical composition of the hot-rolled steel sheet of this embodiment.

  Using the molten steel, an ingot of 200 mm × 200 mm × 200 mm was manufactured by an ingot forming method. Using a 3-stand hot rolling mill, the ingot was rolled into the rolling conditions shown in Tables 2 and 3 (heating temperature, heating time, total rolling reduction MFR, total rolling reduction TFR, final rolling reduction FR, friction coefficient μ, and The hot rolled steel sheet was manufactured by performing hot rolling at a finish rolling temperature FT). After hot rolling, the hot-rolled steel sheet is cooled and wound under the cooling conditions and winding conditions shown in Table 3 (air cooling time, forced cooling rate CR, cooling stop temperature ST, residence time, and winding temperature). Went. The thickness t of the manufactured hot-rolled steel sheet was 2.6 to 4.2 mm.

  In test numbers 4-6, 9, 13, 14, 16, and 17, the hot-rolled steel sheet after rolling was cooled to a coiling temperature CT (° C.) at a cooling rate CR, and then coiled. That is, forced cooling was not stopped midway and no residence time was provided.

[Microstructure observation test]
Based on the measurement method described in the above-mentioned “Measurement Method of Area Ratio of Each Phase in Microstructure”, ferrite area ratio, bainite area ratio, pearlite area ratio, martensite area ratio, and retained austenite of each test number The area ratio was determined. Table 4 shows the obtained area ratio of each phase. “Α” in Table 4 is the ferrite area ratio (%). “B” is the bainite area ratio (%). “P” is the pearlite area ratio (%). “M” is the martensite area ratio (%). “Residual γ” is a retained austenite area ratio (%).

[Band colony area ratio BCR]
Based on the method described in “Method for Measuring Area Ratio BCR of Band-shaped Colony” described above, the area ratio BCR of the band-shaped colony of each test number was obtained. Table 4 shows the obtained results.

[Roughness colony area ratio CCR]
Based on the method described in the above-mentioned “Method for Measuring Area Ratio CCR of Coarse Colonies”, the area ratio of coarse colonies of each test number was determined.

[Measurement method RHv of Vickers hardness ratio]
The Vickers hardness ratio RHv (= HvS / HvC) of each test number was determined based on the method described in the above “Method for measuring Vickers hardness ratio”. Table 4 shows the obtained results.

[Density TN of coarse compound]
Based on the method described in “Method for measuring density TN of coarse compound” described above, density TN (pieces / mm ² ) of coarse compound of each test number was obtained.

[Tensile strength TS and yield ratio YR]
JIS No. 5 tensile test pieces were produced from the hot-rolled steel sheets having the respective test numbers. Using an Instron type tensile tester, a static tensile test based on JIS Z2241 was performed, and yield strength YS (MPa), tensile strength TS (MPa), and yield ratio YR were obtained. Table 5 shows the obtained tensile strength and yield ratio.

[Bendability evaluation test]
Three test pieces of 35 mm × 80 mm × sheet thickness t were sampled from the hot-rolled steel plates of each test number. The longitudinal direction (80 mm) of each test piece was parallel to the rolling direction.

  Each test piece was bent by the V-block method. The vertical angle of the metal fitting was 75 °, and the corner R was 0.5 mm. First, a preliminary bending process was performed to form a bent portion on the test piece, and a contact bending process was further performed. The presence or absence of cracks on the outer surfaces of the bent portions of the three test pieces after the preliminary bending process or the contact bending process was investigated. When cracks were confirmed in 2 of the 3 test pieces, it was determined that the bendability was low (indicated as “NA” in Table 5). On the other hand, when cracks were confirmed in one of the three test pieces (indicated as “G” in Table 5), and when no crack was confirmed in any of the three test pieces (Table No. 5 is represented as “E”), and it was judged that the bendability was high.

[Shear workability evaluation test]
Three test pieces having a width of 60 mm and a length of 25 mm parallel to the rolling direction were sampled from the hot-rolled steel sheets of the respective test numbers.

  Each test piece was sheared. The cutting clearance during shearing was a plate thickness t × 15%. Shearing was performed in parallel with the rolling direction of the test piece, and a 25 mm long × 10 mm wide portion of the test piece was cut.

  The cut end surface of the remaining part (25 mm length × 50 mm width part) of the test piece after cutting was visually observed to check for cracks. Of the three test pieces, when two or more test pieces were confirmed to be cracked, it was judged that the shear workability was low (indicated as “NA” in Table 5). On the other hand, when the number of test pieces in which cracks were confirmed was one or less among the three test pieces, it was determined that the shear workability was high (indicated as “E” in Table 5).

[Test results]
Referring to Tables 1 to 5, the chemical compositions of the hot-rolled steel sheets having test numbers 4, 7, 8, 10, 11, 13, 14, 16, 24, 25, and 28 were appropriate. Furthermore, the heating temperature and heating time were appropriate. Further, the lower limit of the final rolling reduction FR was 20% or more, and the friction coefficient μ was 0.25 or more. Furthermore, the air cooling time was 10 seconds or less, and the air cooling rate CR was 15 ° C./s or more. Furthermore, the coiling temperature CT was in the range of 400 to 600 ° C., which was appropriate. Therefore, in the microstructures of these test numbers, the total area ratio of ferrite and bainite is 89% or more, the pearlite area ratio is 5% or less, the martensite area ratio is 3% or less, and the residual austenite area ratio Was 3% or less. Further, the Vickers hardness ratio RHv was 0.80 or less. Therefore, in these test numbers, the tensile strength TS was 590 MPa or more, and the yield ratio YR was 0.75 or more. Furthermore, in the bendability evaluation test, the hot-rolled steel sheets having these test numbers exhibited excellent bendability.

  Furthermore, in test numbers 7, 8, 10 and 11, the total reduction ratios MFR and TFR and the finish rolling temperature FT were appropriate in the production conditions, and the air cooling time was within a range of 3.0 to 10 seconds. . Therefore, the band-like colony area ratio BCR of these test numbers was 40% or less, and the area ratio CCR of the coarse colony was 30% or less.

  Therefore, the hot rolled steel sheets of test numbers 7, 8, 10 and 11 were further excellent in shear workability. The total rolling reduction MFR of test number 16 exceeded 30%. Therefore, although it was inferior to test number 7, 8, 10 and 11, favorable shear workability was obtained.

  On the other hand, in test number 1, the C content was too high. Therefore, the martensite area ratio was higher than 3%, and the Vickers hardness ratio exceeded 0.80. Therefore, the bendability was low.

  In test number 2, the P content and the S content were too high. Therefore, bendability and shear workability were low. Since P segregated, the shear workability was lowered, and the bendability was thought to be lowered due to the formation of S inclusions.

  In test number 3, the Ti content was too high. The bendability was low. This is probably because coarse Ti carbonitride was produced.

  In Test No. 5, although the chemical composition was appropriate, the friction coefficient μ was too low. Therefore, the Vickers hardness ratio RHv exceeded 0.80. Therefore, the bendability was low. In test number 5, the total rolling reduction MFR was too low. Therefore, the band-like colony area ratio BCR exceeded 40%, and the shear workability was low.

  In test number 6, although the chemical composition was appropriate, the final rolling reduction FR was too low. Therefore, the Vickers hardness ratio RHv exceeded 0.80 and the bendability was low.

  In Test No. 9, although the chemical composition was appropriate, the final rolling reduction FR and the friction coefficient μ were too low. Therefore, the Vickers hardness ratio RHv exceeded 0.80 and the bendability was low.

  In test number 12, although the chemical composition was appropriate, the heating temperature was too low. Therefore, the tensile strength was less than 590 MPa.

  In Test No. 15, the chemical composition was appropriate, but the air cooling time was too long. Therefore, the tensile strength was less than 590 MPa.

  In test number 17, although the chemical composition was appropriate, the coefficient of friction was too low. Therefore, the Vickers hardness ratio RHv exceeded 0.80 and the bendability was low.

  In test number 18, although the chemical composition was appropriate, the coiling temperature was too low. Therefore, the retained austenite area ratio exceeded 3%. As a result, the yield ratio was less than 0.75 and the bendability was low.

  In test number 19, the Si content was too low. Therefore, the pearlite area ratio exceeded 5% and the bendability was low.

  In test number 20, the Mn content was too low and the P content was too high. Therefore, the yield strength was too low and the bendability was low.

  In test number 21, the C content was too low. Therefore, in the test number 21, the tensile strength TS was less than 590 MPa.

  In test number 22, the S content and the N content were too high. Therefore, the bendability was low.

  In test number 23, the finish rolling temperature FT was too low. Therefore, the band-like colony area ratio BCR exceeded 40%, and the shear workability was low.

  In test number 25, the cooling rate CR was too slow. Therefore, the yield strength was low.

  In test number 26, the coiling temperature CT was too high. Therefore, the Vickers hardness ratio RHv exceeded 0.80 and the bendability was low.

  In test number 27, the coiling temperature CT was too low. Therefore, the martensite area ratio was higher than 3%, and the Vickers hardness ratio exceeded 0.80. Therefore, the bendability was low.

  The embodiment of the present invention has been described above. However, the above-described embodiment is merely an example for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiment, and can be implemented by appropriately changing the above-described embodiment without departing from the spirit thereof.

Claims (4)

A hot-rolled steel sheet,
% By mass
C: 0.01 to 0.2%
Si: 0.01 to 2.5%,
Mn: 0.5 to 3.0%
P: 0.02% or less,
S: 0.005% or less,
Sol. Al: 0.02 to 0.5%,
Ti: 0.02 to 0.25%,
N: 0.010% or less,
Nb: 0 to 0.1%,
V: 0 to 0.4%,
Mo: 0 to 0.4%,
W: 0 to 0.4%
Cr: 0 to 0.4%, and
A total content of Ca, Mg and rare earth elements (REM): 0 to 0.01%, with the balance being a chemical composition consisting of Fe and impurities;
Microstructure composed of ferrite and bainite having an area ratio of 89% or more, pearlite having an area ratio of 0 to 5%, martensite having an area ratio of 0 to 3%, and retained austenite having an area ratio of 0 to 3%. When,
With a tensile strength of 590 MPa or more,
A hot-rolled steel sheet in which the Vickers hardness HvC at the center of the thickness of the hot-rolled steel sheet and the Vickers hardness HvS at a position 100 μm deep from the surface of the hot-rolled steel sheet satisfy Expression (1).
HvS / HvC ≦ 0.80 (1) The hot-rolled steel sheet according to claim 1,
When the thickness of the hot-rolled steel sheet is tmm, a crystal colony having a {112} <110> orientation and a crystal orientation of 10 ° or less at the center of the thickness within ± 0.12 t from the thickness center. A ratio of the total area of crystal colonies having a {112} <110> orientation and a crystal orientation of 10 ° or less and an aspect ratio of 0.35 or less to a total area of 40% or less. The hot-rolled steel sheet according to claim 1 or claim 2,
The chemical composition is
Nb: 0.002 to 0.1%,
V: 0.01 to 0.4%
Mo: 0.01 to 0.4%,
W: 0.01 to 0.4%, and
Cr: A hot-rolled steel sheet containing one or more selected from the group consisting of 0.01 to 0.4%. The hot-rolled steel sheet according to any one of claims 1 to 3,
The said chemical composition is a hot-rolled steel plate which contains 0.0002-0.01% of 1 type, or 2 or more types selected from the group which consists of Ca, Mg, and REM in total.